The changes in energetic, structural, and electronic properties of the metallic (5,5) single-walled carbon nanotube (SWNT) with the degree of sidewall covalent functionalization Of CCl2 are investigated extensively by using density functional theory calculations. The saturation concentration Of CCl2 covalent functionalization is predicted to be 33.3%. The cycloadducts always adopt an open structure. A band gap opens as the functionalization concentration reaches 11% and then basically increases with increasing functional ization concentration. These results are in agreement with available experiments and can be applied to accurately predict the band gap of metallic SWNTs produced by the HiPco method at a given CCl2 functionalization concentration.